It has been known for a long time to produce electron beam diffraction patterns to determine the structure of crystals or polycrystalline material. The interferences which occur when electron beams pass through crystals or through polycrystalline materials or are reflected in crystals, particularly Laue interferences in monocrystals and Debye-Scherrer rings in polycrystalline materials, allow more exact conclusions concerning the structural composition of the examined materials.
Whereas the examination of the structure of inorganic, particularly mineral materials, using electron beam diffraction patterns has produced genuine results concerning the crystalline structure of these materials, particular difficulties have arisen in this respect during the examination by electron beam diffraction of the structure of organic and biological substances or, generally speaking, during the examination of natural and synthetic materials which have a periodic structure and which are relatively sensitive and have a relatively poor heat-conductivity. This is because considerable structural changes occur as a result of irradiating these substances with electrons of the energy densities necessary to produce an image. These structural changes are referred to below as beam damage. Much has been written in technological literature about undesired structural changes of this type, particularly by L. Reimer in the periodical "Lab. Invest." 14, 1082 (1965), by K. Stenn and G. F. Bahr in the periodical "J. Ultrastruct. Res." 31, 526 (1970), by R. M. Glaeser in the book "Physical Aspects of Electron Microscopy and Microbeam Analysis", page 205, published by Wiley and Sons New York 1975, and edited by B. M. Siegel and D. R. Beaman, and by W. Baumeister, M. Hahn and U. P. Fringeli in "Zeitschrift fur Naturforschung" 31c 746 (1976).
As has been experimentally established by G. Siegel in "Zeitschrift fur Naturforschung" 27a, 325 (1972), the crystalline arrangement of a paraffin crystal is considerably altered at a current density of the electron beam of 8 electrons per square Angstrom at an accelerating voltage of the electron beam of 100 kV. Similar experiments in adenosine crystals, which R. M. Glaeser described in the periodical "J. Ultrastruct. Res." 36, 466 (1971), have shown that the crystalline arrangement of an adenosine crystal is almost completely destroyed at a current density of the electron beam of 6 electrons per square Angstrom at an accelerating voltage of 80 kV.
The problem arising in respect of these results in the production of electron beam diffraction patterns using photographic films becomes clear when it is considered that a current density of one electron per .mu.m.sup.2 is necessary to blacken a photographic film using electrons. This means that with a resolution of 5 Angstrom, which corresponds to a magnification .times.100,000, it is necessary to irradiate the object, i.e., the organic or biological substance which is to be examined, with 100 electrons per square Angstrom when the primary electron beam has a diameter in the order of 1 .mu.m in the object plane.
Because of this problem it has hitherto been impossible to obtain electron beam diffraction patterns except for relatively insensitive substances, without producing noticeable changes of the original structure of the substances. Thus, as a result of the previously mentioned beam damage, experiments involving examining sensitive substances by means of electron beam diffraction patterns do not allow precise conclusions concerning the original unchanged structure of those substances.
Since the undesired structural changes are attributable to the fact that the energy which is supplied to the examined organic and biological substances by the electron beam cannot be dissipated to the surroundings fast enough by these substances, there is a possibility of obtaining electron beam diffraction patterns of these sensitive substances by a method involving cooling the substances to a very low temperature during their examination, that is to a temperature of 4.degree. K. using liquid helium. Such a method, using a 400 kV-electron microscope comprising a superconductive objective lens, has developed by the laboratory of Siemens AG, Munich and has been reported by J. Dietrich, F. Fox, E. Knapek, G. Lefrane, R. Nachtrieb R. Weyl and H. Zerbst in the periodical "Ultramicroscopy" 2, 241 (1977). However, for this purpose a high degree of technical complexity is required.